Exhaust-gas turbochargers are increasingly used for increasing power in internal combustion engines, in particular in motor vehicles. This is done ever more commonly with the aim of reducing the structural size and weight of the internal combustion engine while maintaining the same level of power or even achieving an increased level of power, and at the same time reducing consumption and thus CO2 emissions, in the context of ever more stringent legal regulations in this regard. The operating principle includes utilizing the energy contained in the exhaust-gas flow to increase the pressure in the intake tract of the internal combustion engine and thereby realize improved charging of the combustion chamber with air/oxygen to thus be able to convert more fuel, for example gasoline or diesel, per combustion process, that is to say increase the power of the internal combustion engine.
For this purpose, an exhaust-gas turbocharger has a turbine, which is arranged in the exhaust-gas tract of the internal combustion engine and which has a turbine rotor driven by the exhaust-gas flow, and a compressor, which is arranged in the intake tract and which has a compressor rotor which builds up the pressure. The turbine rotor and compressor rotor are fastened rotationally conjointly to the opposite ends of a rotor shaft and thus form the turbocharger rotor, which is rotatably mounted by means of its rotor shaft in a bearing unit arranged between turbine and compressor. Thus, by means of the exhaust-gas mass flow, the turbine rotor, and via the rotor shaft which is in turn the compressor rotor, is driven, and the exhaust gas energy is thus utilized for building up pressure in the intake tract.
Turbines and compressors are turbomachines and, owing to the laws of physics, have an optimum operating range in a manner respectively dependent on structural size and design, which optimum operating range is characterized by the mass throughput, the pressure ratio and the rotational speed of the respective rotor.
By contrast to this, the operation of an internal combustion engine in a motor vehicle is characterized by dynamic changes of the load and of the operating range.
To now be able to adapt the operating range of the exhaust-gas turbocharger to changing operating ranges of the internal combustion engine and thus ensure a desired response behavior as far as possible without noticeable decelerations (turbo lag), exhaust-gas turbochargers are equipped with additional functions, such as for example so-called variable turbine geometries (VTG) or wastegate devices (WG) on the exhaust-gas or turbine side and overrun air recirculation or blow-off devices on the air feed or compressor side. These serve for minimizing the inert behavior and thus the decelerated response behavior of the turbocharger and avoiding damaging operating states.
It is also known to use combinations of multiple turbochargers in a parallel or sequential arrangement or to use additional compressors which are operated mechanically or by electric motor, so-called supercharging blowers or superchargers, in order to cover the various operating conditions of the internal combustion engine, in order to efficiently increase the power in all rotational speed ranges and in particular during acceleration processes, and in particular to avoid the undesired turbo lag, which is caused by excessively low charge pressure in low rotational speed ranges of the turbocharger in conjunction with the inertia of the turbocharger rotor.
A supercharging device of this type, which has a conventional exhaust-gas turbocharger and an auxiliary compressor arranged in the fresh-air mass flow in series or in parallel with respect to the turbocharger compressor, which auxiliary compressor has a drive independent of the exhaust-gas flow, for example an electric motor drive, is disclosed, for example, in DE 100 23 022 A1.
By contrast, in operating phases in which the power of the internal combustion engine is decreased quickly, it is the case, likewise owing to the inertia of the turbocharger, that an excess of compressor power exists, which may lead to so-called compressor surging. Compressor surging refers to an operating state in which air that has already been compressed flows back from the high-pressure side of the compressor via the compressor rotor in periodic surges and thus generates undesired oscillations in the intake tract. To avoid such operating states, exhaust gas is conducted, so as to bypass the turbine of the turbocharger, into the exhaust-gas tract, for example by means of a wastegate device, and already-compressed fresh air is blown off downstream of the compressor or is expanded across an overrun air recirculation device and recirculated into the intake region. The arrangement and functioning of an overrun air recirculation valve of this type is known, for example, from documents DE 28 23 067 C2 and DE 197 12 850 A1.
In this way, the available energy is discharged, unutilized, into the surroundings, which has an adverse effect on the overall energy balance and thus on the efficiency of the internal combustion engine.